Monofilament-reinforced hollow fiber membrane with scalloped lumen

ABSTRACT

A hollow fiber membrane is formed by embedding a braid having a spiral open weave of monofilaments only, to avoid a “whiskering” problem. The open weave is characterized by contiguous, circumferential, rhomboid-shaped areas of polymer film separated by monofilaments. When the braid is supported on a plasticized PVA cable having a scalloped periphery, the braid can be infiltrated with membrane polymer which, when coagulated, embeds the braid positioning it around the lumen. The embedded spiral weave, free of any circumferentially constricting monofilament, allows the membrane to be biaxially distensible. The membrane has “give” not only in the axial or longitudinal direction but also in the radial direction. “Give” in the radial direction permits soiled membranes to be backwashed under higher pressure than in a comparable braid which is not radially distensible.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.13/659,648, filed Oct. 24, 2012, the contents of which are incorporatedherein in their entirety by reference. U.S. patent application Ser. No.13/659,648 is a continuation-in-part of U.S. patent application Ser. No.13/338,557, filed Dec. 28, 2011, now U.S. Pat. No. 8,827,085, whichclaims priority to and the benefit of Korean Patent Application No.10-2011-0134597, filed Dec. 14, 2011, and Korean Patent Application No.10-2011-0039181, filed Apr. 26, 2011. U.S. patent application Ser. No.13/659,648 also claims priority to and the benefit of PCT/KR2012/003189,filed Apr. 25, 2012.

FIELD OF THE INVENTION

This invention relates to a macroscopic composite hollow fiber membranewhich is reinforced with multiple multifilament yarns braided into atubular shape; each such yarn is made by plaiting or twisting multiplestrands or monofilaments of a polymer having desired tensile strength.The tubular braid is then coated with a membrane-forming dope (polymersolution or “dope” for brevity) which is coagulated to form a tubularpolymer membrane having a lumen diameter of at least 0.5 mm. Suchconstruction is designed for use in demanding applications in whichpermeate is required to meet strict specifications. For example, forwater filtration, one or more skeins are used, each skein comprising amultiplicity of hollow fiber membranes (hereafter “fibers” or“membranes” for brevity) in a module which is deployed in a reservoir of“dirty” water to be filtered. A “skein” of fibers is a bundle of fibersdeployed adjacent one and another, all in the same general direction. A“module” is a skein of fibers opposed ends of which are secured inheaders, typically by potting. Multiple modules are typically deployedin a reservoir containing a large amount of liquid to be filtered, e.g.in municipal water filtration plants. When, during filtration, thepressure drop through the fibers in a module gets sufficiently high, themodule is backwashed with permeate under pressure. Desirable systemsprovide permeate economically, by providing high permeate flow and byminimizing damage to the hollow fiber membranes.

BACKGROUND OF THE INVENTION

Braided hollow fiber membranes are commonly used in modules containingfrom several hundred to several thousand membranes. Damage to a singlemembrane in a module, which damage results in dirty water contaminatingthe permeate, is a serious problem which occurs more often than desired.Though the permeate is typically water, the permeate may be anyfilterable liquid to be separated from a suspension or dispersion offinely divided particles in the liquid.

To date, numerous braided membranes have been disclosed, each of whichpurports to provide desirable filtration efficiency but offer scantuseful knowledge relating to avoiding the damage to a membrane ormaximizing permeate efficiency. Emphasis on physical strength of themembrane is embodied in disclosures of U.S. Pat. Nos. 3,644,139;4,061,821; 5,472,607; 5,914,039; 6,354,444; 7,165,682; 7,172,075;7,306,105; 7,861,869; 7,909,172 and others.

The requirement of strength decreed that these prior art braids be madeby braiding multiple yarns, each comprising lengths of multiplemonofilaments (or “filaments” for brevity). The drawbacks of usingmultifilament yarns were either overlooked or ignored.

The method of making a tubular hollow fiber membrane decreed that thecross-section of the lumen be circular. Whether such a fiber is made bycoating a woven tubular reinforcement with polymer, or extruded with awoven circular reinforcement embedded in the fiber's wall, thecross-section of the lumen remained circular.

Membranes such as are disclosed in U.S. Pat. No. 4,061,821 to Hyano etal, (hereafter '821) have braid embedded beneath a thick polymer film toprovide a stabilizing effect during use of the membrane. The term“embedded” as used herein describes yarn or monofilament with at least99% of its surface coated with polymer. Braid having an inner diameterin the range of 0.5-10 mm and unspecified “thin thickness” is made fromfilaments overlying and randomly overlapping one and another (see FIGS.4, 5 & 6 in '821) in multiple layers but are preferably made frommultifilament yarn. The stabilizing effect of the openings in thereinforcing material was lost (see sentence beginning at the bottom ofcol 4, and bridging cols 4 and 5) when the braid was coated withpolymer, so that their reinforced membrane was not an effectivemembrane.

The problem of stability was addressed in U.S. Pat. No. 5,472,607 toMahendran et al, (hereafter '607) which teaches a film having a wallthickness in the range from 0.01 mm to 0.1 mm, supported on the outercircumferential surface of a preshrunk braid; a major portion of thearea of the circular cross-section of the porous tubular support, viewedalong the longitudinal central axis, is free from film and not embeddedin film. Thus, it was not known how embedding the braid in the filmaffected the performance of the membrane; nor was it established whetherfailing to embed the braid in the film provided a significant advantage.

Membranes such as are disclosed in U.S. Pat. No. 6,354,444 to Mahendranet al, (hereafter '444) are produced by first weaving a tubular braid ofmultifilament yarn to have a cylindricity>0.8, preshrinking the braid,then coating the outer circumferential surface of the cylindrical braidwith a dope of polymerizable membrane-forming polymer. The term“cylindricity” (sometimes referred to as “roundness”) refers to howperfectly the circular cross-section of the tubular support matches thegeometry of a true circle drawn to correspond to the mean diameter ofthe braid, a perfect match being 1.0. By “weaving” is meant that thefilaments are interlaced without being knotted (as they would be if thebraid was knit). “Dope” refers to fluid “membrane polymer”, e.g. polyvinylidene fluoride (“PVDF”) whether molten or in a solution. If insolution and coagulated, the dope forms a film having a wall thicknessof>0.2 mm and with desired attributes for the filtration of fluid to befiltered, typically dirty water. The '444 braid is relatively dense,having an air permeability in the range of from 1 to 10 cc/sec/cm² at1.378 kPa so that the voids in the braid are small enough to providesubstantial resistance to the passage of air, and thus inhibitsubstantial penetration of polymer. The braid is preshrunk to providestability of the braid. Yarn lying in a generally longitudinalorientation (along the z-axis) provides extension at break of theuncoated braid of at least 10% which extensibility is referred to as“give”.

The weave of the '444 braid is a circularly woven tubular braid. Suchbraids are tightly woven with at least one circumferential yarn lying ina generally x-y plane (z-axis is longitudinally axial). This orientationnecessarily constricts and prevents radial distension of the braid, butthe preshrunk braid does have “give” in the longitudinal direction.However, when the braid is coated with a relatively elastic polymer toform the membrane, it is essentially longitudinally non-extensible(along the z-axis). In other words, the '444 membrane, whether pulled inthe axial direction or pressured from within during backwashing, hasessentially no “give”. The importance of “give” relates particularly toeffective backwashing. The higher the backwashing pressure the better,if it does not damage the membranes, because such pressure allows fasterand more effective cleansing of contaminated membranes and thereforeprovides an economic advantage.

Because the '444 braid is deliberately not embedded in the polymer, yarndefining the lumen (bore) of the membrane is not coated with polymer.Other references disclose braids woven to minimize the problem oftoo-deep penetration of the polymer film. The non-embedded yarn, in allsuch instances, is prone to damage such as pin-holes. Such damagelessens the initial high bubble point of the freshly deployed membrane.The “bubble point” refers to the pressure under which a stream of airescapes through the largest pore in a wall of a wetted, defect-freemembrane which has desirable flux. Further, the importance of stabilityof the structure of the braid during operation, particularly the effectof shrinkage, was not known.

Though U.S. Pat. No. 7,861,869 discloses a semipermeable capillarymembrane reinforced by a braid of monofilament yarn, the yarn is made bybundling multiple monofilaments (36 in example 1). The braid is not madeby braiding separate monofilaments. Penetration of the dope into thebraid is controlled so that the inner channel (lumen) of the braid isnot blocked. The process taught herein prepares an “outer skinned”version of the reinforced membrane, explicitly avoiding embedding thebraid.

WO-A-0397221A1 describes a tubular fiber membrane which islongitudinally reinforced by unbraided yarns, not by a braid. The axialbore is formed by injecting an internal coagulation solution in thecentre but the thickness of the annular film defining the lumen cannotbe controlled.

US 2009/0206026 A1 to Yoon et al, titled “External pressure type hollowfiber membrane having reinforcing supporter with monofilament for gasseparation and water treatment, and method and apparatus for preparingthe same” states: “The hollow fiber membrane of the present inventionhas excellent pressure resistance and high tension force by using therigid and tubular supporter, an improved softness by using themonofilaments, and an increased bonding force between the supporter andthe coating layer by increasing the concave-convexo degree of thereinforcing supporter.” (see '026 Abstract, lines 6-11). That thetubular supporter in the described hollow fiber membrane is rigid, isreiterated under “Industrial Capability” (see line 3 of paragraph[0057]. Such rigidity serves to distinguish the '026 membrane over themembrane of '607 to Mahendran et al, discussed in '026 as being the mostrelevant reference which teaches that “The support itself is so flexible(flaccid) that it does not have a circular cross-section and collapseswith finger pressure.” (see Abstract, lines 4-6) “By “flaccid” is meantthat the denier of monofilaments used in the yarns or “ends” forcarriers which are braided, and the number of picks/unit length of thebraid, are such that a tubular braid has very little mechanical strengthin a vertical plane normal to its longitudinal central axis, so that itis so flexible that it can be easily manually tied into a knot. Atypical braid starts out as multiple filaments which make up a single“end” and two “ends” are plied together in 3.8 twists/25.4 mm to make upa yarn or “carrier”. Multiple carriers, preferably 24, are used to braida tubular braid.” (see '607, col 3 lines 24-33). Clearly, the '026statement relating to a rigid and tubular supporter are meant todistinguish over the '607 braid.

Note that though FIGS. 4 and 5 in '026 purport to be photomicrographs ofthe reinforcing supporter, both woven with monofilaments of 130 deniersand of 32 and 24 yarns respectively, other than stating that thediameter of the supporter can be controlled according to the number ofcones (see [0042]), there is no indication in either photomicrograph asto the diameter of the woven braids shown. Neither is there anyidentification, anywhere, either of the weave, or of the machinery, usedto make a braid having any specified diameter, much less a nominalinside diameter in the range from about 1.0 mm (to make a membranehaving a nominal outer diameter of 1.5 mm, depending upon the denier ofthe monofilament to be used), to about 2.5 mm (to make a membrane havinga nominal outer diameter of 3.0 mm, depending upon the denier of themonofilament to be used), as specified for the braid and membraneclaimed herein. In particular, there is no mention of using a flexiblecable, dissolvable in an aqueous solution (referred to as“aqueous-dissolvable”) upon which to weave the braid. By “nominal” ismeant “average”.

Particularly noteworthy is that the '026 membrane is woven with both,monofilament and multifilament yarns; this provides convincing evidencethat the inventors of the '026 braid failed to realize that “whiskering”and “fuzz” were the root causes of failure in membranes withmultifilament braids.

In FIG. 6 of '026 there is illustrated an automatic device in which aperforated wire 2 extends along the central vertical axis of an injectorfor an internal coagulating solution 4. A high pressure injection nozzle3 injects the internal coagulating solution onto the wire, and thesolution is also squirted through the perforations while the reinforcingsupporter passes over and is forwarded by the roller 5 in contact withthe wire 2. (see [0045]).

Aside from the problem of axially perforating about a 2.0 mm diameterwire, doing which is beyond the skill of the inventors herein, it willbe seen in the test presented in example 1 below, that an open weavetubular braid having the diameter claimed herein, made with wovenmonofilament in the size range claimed herein, cannot be forwarded (or“passed”) over a wire as described in '026 because the friction is toogreat, and other reasons. Numerous attempts to forward a tubular braidof monofilaments only (see example 1 below) to make a membrane in therange of nominal outer diameters from 1.5-3.0 mm, fails to produce ausable, undistorted, uniform membrane. The '026 reference is therefore anon-enabling disclosure. Moreover, manually pulling the braid over thewire after the braid is coated with coagulant polymer, results indestruction of the membrane, again, because of the flaccid membrane andits excessive friction. To make and use the membrane claimed in '026would require undue experimentation.

US 2004/0197557 to Eshraghi et al teaches (a) providing a moltenremovable substrate material in the form of an extrudate of a moltenpolymeric material (see [0011] to make a hollow fiber membrane having adissolvable core, and the use of reinforcing fibers as follows:“Additionally, one or more reinforcing fibers can be incorporated intosuch polymeric membrane to form a fiber-reinforced tubular polymericmembrane structure. Preferably, such reinforcing fibers extendcontinuously along the longitudinal axis of the fibrous core orsubstrate and therefore provide axial reinforcement to the hollowfibrous membrane. Fiberglass having an average diameter of about 0.1-500μm is particularly suitable for practice of the present invention, whileother fibrous materials, including but not limited to carbon fibers,metal fibers, resin fibers, and composite fibers, can also be employedfor reinforcing the hollow fibrous membrane. The reinforcing fiber caneither be co-extruded with one of the polymeric membrane-forming layers,or be encapsulated between two polymeric membrane-forming layers, toform an integral part of the hollow fibrous membrane.” (see [0085])There is no suggestion, beyond the statement that “such reinforcingfibers extend . . . hollow fibrous membrane” how one or more reinforcingfibers are to be incorporated into the polymeric membrane.

The '557 publication states “the solid core fiber itself is formed of asolid-phase removable substrate material, and at least one layer of apolymeric membrane-forming material is coated directly onto such solidcore fiber.” (See [0023]). It thereafter states: “the molten removablesubstrate material is co-extruded with the membrane-forming polymer.”(See [0041]-[0047]).

It is clear that Eshraghi et al did not extrude PVA because it degradesbefore it can be melt-extruded, irrespective of what grade of PVA isused. As evident in example 2 below, attempts were made to extrude eachof three grades of PVA available from Kuraray, namely fully hydrolyzed(F-05 and F-17); intermediate hydrolyzed (M-17); and partiallyhydrolyzed (P-24, P-20, P-17 and P-05). The temperature at which each ofthe polymers degrades is lower than its softening temperature. Thereforeeach attempt resulted in severe degradation of each.

In the description of the process illustrated in FIG. 3A, the '557publication states “a string or a tow of removable core fiber 122 from aspool 120 is passed through the extrusion die 124. A thin layer of theviscous polymeric solution 101 is therefore applied onto the removablecore fiber 122, forming a coated fiber 132.” ([See 0086]) The core fiber122 could not have been flexible PVA as it necessarily would have to beplasticized with just sufficient plasticizer to provide a core fiberwhich was not degraded.

A core made from PVA in a solution of hot water has insufficientstrength to maintain its cylindrical form—discovered to be a criticalrequirement for making the open weave braid of this invention. Not beingable to make a PVA core negated the suggestion in '557 that PVA may beused for the core.

As for the reinforcing, the '557 publication states “The reinforcingfiber can either be co-extruded with one of the polymericmembrane-forming layers, or be encapsulated between two polymericmembrane-forming layers, to form an integral part of the hollow fibrousmembrane.” There is no suggestion that the extrudate be covered with abraid before being coated with membrane polymer, and no way this couldbe accomplished using the teachings of their disclosure.

In a manner analogous to that stated above, commercially available ethylvinyl alcohol (EVOH) from Kuraray; commercially available polylacticacid from Nature Works; commercially available nylons from Shakespeare;and no-longer commercially available copolyester from Eastman, failed toproduce a usable core despite numerous trials in each of which theconditions of extrusion were changed.

Clearly the disclosure of the '557 publication is not an enablingdisclosure.

With respect to the use of monofilaments, apart from a braid thereof,the '557 publication provides no indication that it recognized the illeffects of “whiskering” and “fuzz” in a braid made with at least somemultifilament yarns.

Neither did the '557 publication recognize that only a tubular braidembedded near the inside diameter of the membrane, so as to reinforcethe lumen, provided the highest peel strength, bubble point and permeateefficiency. There is no suggestion that a braid be woven using onlymonofilaments woven in a particular way, namely with an open weave so asto avoid having circumferentially restricting filaments which would nothave “give” under abnormally high backwashing pressure.

Publication No. WO/2010/148517 to Cote et al (hereafter the “'517publication”) presents the concept of using a “dissolvable filament(solid or hollow) core” to make a hollow fiber membrane (see [0040]). Itstates that “the core can be a solid or capillary tube can be laterdissolved in a solvent, preferably the solvent used to coagulate themembrane (typically water). Examples of water-soluble polymers includePVA, EVOH (made by Kuraray), as well as some forms of polyester(available from Eastman) and nylon (available from Shakespeare).” (see[0065]). Not mentioned is highly amorphous vinyl alcohol (HAVOH) andmore commonly available polylactic acid (PLA), cellulose acetate,hydroxyethyl cellulose, polyethylene oxide (PEO) and polyethylene glycol(PEG), all of which are water-soluble. If he had used PVA as a removablecore, he would have realized that despite extended washing with water,more than 10% of the usable pores in the lumen of the membrane remainclogged, and the membrane requires a wash with an aqueous solvent inwhich the PVA is far more soluble than in water. They would havedisclosed such a complex cleaning requirement.

The Problem

Currently used braided multifilament membranes are more readilysusceptible to damage than they should be, resulting in leakage. Theinventors herein discovered that such damage, resulting in leaks,typically occurs at vulnerable “thin spots” where yarns overlap near thesurface of the braid; further, that such overlap of multifilament yarnsor broken yarn is also conducive to formation of “whiskers” or “fuzz”which initiates pin-hole leaks, resulting in a low bubble point. Sincein the '444 patent, thin film is deliberately restricted to the upperportion of the tubular braid, the uncoated lumen is formed andreinforced with unprotected, braided yarn, prone to whiskering. Suchyarn tends to trap contaminant particles entering with the backwash andbroken whiskers contaminate the permeate.

Still further, the relatively greater thickness of multifilament braid,relative to the thin film of polymer overlying the surface of the braid,results in a non-uniform thin polymer film having poor adhesion and avariable, low peel strength. In those instances where the lumen of thebraid is deliberately coated with polymer film, the annular thickness ofpolymer film is uncontrollable (as evident in the '607, '869 and '075patents inter alia). Though the '517 publication presented the conceptof using a dissolvable polymer core such as PVA, what remained to bediscovered was (i) how to overcome the degradation problem of PVA, yetmake a reliably uniform core, (ii) how to make a flexible non-degradedPVA core which is strong enough to withstand the forces required to makean open weave braid snugly supported on the core, (iii) how to avoidtrapping air in the membrane, (iv) how to overcome the problem ofdissolving the PVA core within a reasonably short time, and (iv) how toensure that upon solving the prior two problems, the resulting membranewould have unclogged pores. Clogged pores would greatly diminish thepermeate efficiency of the membrane. The goal was to obtain higherpermeate efficiency than that obtained with multifilament reinforcedmembranes, and to remedy the aforementioned problems of braidedmembranes exemplified by the '517, '444 and '177 membranes.

In addition, particularly because in a skein having reinforced fiberswith a nominal diameter>1.5 mm, drawing a vacuum on the skein tends tocollapse some fibers, it is also a goal to provide a monofilamentreinforced fiber with reinforcement which improves on the strength ofprior art reinforced fibers.

SUMMARY OF THE INVENTION

It has been discovered that using only monofilaments (“filaments”), andeliminating the use of multifilament yarn, produces an unexpectedlysuperior braided membrane whether its lumen is cylindrical or notcylindrical. Multiple filaments are woven (interlaced, plaited orbraided) directly upon an aqueous-dissolvable solid core or “cable” ofnecessarily plasticized poly vinyl alcohol (“PVA”), plasticized with aplasticizer in an amount sufficient to yield a homogeneous, solid,flexible extrudate having a density+10% of the density of PVA andsufficient strength to provide an elongated, continuous, solid supportfor a braid of monofilaments woven on the surface of the extrudate. ThePVA is preferably plasticized with from 5-20% by weight of a plasticizerchosen from polyethylene glycol (“PEG”), polypropylene glycol (“PPG”),ethylene oxide capped polypropylene glycol (“EO capped PPG”), sorbitol,glycerol, ethylene glycol, polyvinyl pyrrolidone (“PVP”),pentaerythritol, 1,4-monoanhydrohexitol, 1,4-3,6-dianhydrohexitol andcopolymers of poly vinyl acetate. Without the critical amount ofplasticizing, the cable would degrade when melt-extruded; would not havethe flexibility to survive further processing, or, the requisitestrength to withstand the forces of weaving from 6-20 monofilaments onthe core's surface; and, if not essentially completely soluble, thatis>99% soluble, in an aqueous cleaning bath, the membrane, when formed,could not be parted from the cable.

Whether the lumen is to be cylindrical or non-cylindrical, the cable isa unitary solid preferably formed by extrusion of the plasticized PVA asa continuous extrudate of arbitrary length, having a cross-section whichmatches that of the desired lumen to be formed by the extrudate. Whencylindrical, the solid continuous cable has a diameter “dc” smaller thanthe nominal outer diameter of the membrane to be made (“dm”) by twicethe thickness “dt” of the wall of the membrane; that is, dc=dm−2 dt, anddm is in the range from 0.75-2.6 mm, and dt is in the range from 0.2-0.6mm. The flexibility of the cable is such that a plasticized cable withdc=2.0 mm can be wrapped around a cylinder 1 m in diameter at 25° C.without breaking.

When non-cylindrical, the lumen may have a scalloped cross-section ofplural interconnected arcuate segments including adjacent, outwardlyconvex arcs, or segments of contiguous circles, interconnected, one toanother at their circumferences. Interconnected, closely spaced apartarcs, defining lobes of a lumen, are preferably generated as portions ofcircles having the same diameter. The arcs are preferably symmetricallydisposed about the x-y axes in a vertical plane which is at right angleto the longitudinal axis of the lumen. The minimal distance betweenadjacent arcs, even in a double-barreled lumen, is less than 25% of thediameter of the circles (and arcs) defining the lobes (see FIG. 5); inlumens having more than two lobes, the arcs defining adjacent lobes ofeach lumen, will typically intersect (see FIGS. 3 & 6).

A lumen with a scalloped cross-section refers to one having a peripheryreminiscent of a scallop's shell, comprising plural interconnectedcircumferential segments. Though such a lumen may be formed by usingplural cylindrical elements of arbitrary length, which have been fused,adhered or otherwise secured to each other longitudinally, at pointsalong their circumferential lengths to form a fluid-tight bond, it ismost preferred to form a solid unitary cable, with the desired number ofradially protruding scallops or lobes, by extruding the plasticized PVA.

When the lumen's cross-section is formed by two interconnected circularsegments to form two lobes, they preferably have the same diameter andare diametrically oppositely disposed relative to each other, the lumenso formed being referred to as a double-barreled lumen.

When the lumen's cross-section is formed by three or more, preferablyfrom 3-6, most preferably 3-4, interconnected circular segments, theypreferably have the same diameter and are radially equiangularlydisposed relative to each other. In a lumen formed with threeinterconnected circular segments, the diameter of each is located 120degrees apart relative to another contiguous circular segment, the lumenso formed being referred to as a three-lobe lumen.

In a lumen formed with four interconnected circular segments, thediameter of each is located 90 degrees apart relative to anothercontiguous circular segment, the lumen so formed being referred to as afour-lobe lumen.

The “lobe profile” of outwardly convex arcs of the lumen is chosen as afunction of the desired ratio of (the depth of polymer “hv”, measured asthe radial distance from the surface of the membrane) to (the nadir ofthe valley between convex surfaces, and “hc”, measured as the radialdistance from the surface of the membrane to the apex or crest of alobe's convex surface), or hv/hc. For all fibers having a scallopedlumen, the ratio of hv/hc>1. (see FIG. 7)

The filaments are spirally woven on the cylindrical surface of the cableto form an open, tubular braid free of a restricting circumferentialfilament in the x-y plane. By “open” is meant that the braid has an airpermeability much greater than 10 cc/sec/cm² at 1.378 kPa because thebraid has essentially no resistance to air passing through it, thereforeensures thorough infiltration and embedding of the braid in the liquidmembrane polymer.

The average diameter of the cable is chosen to match the cross-section,more specifically, to match the hydraulic diameter of the lumen of themembrane to be made. The hydraulic diameter HD=4A/ρ where A is the areaof the lumen, measured in very small increments by computer, and ρ isthe linear measurement of the periphery of the lumen, measured in verysmall increments by computer. HD may be in the range from about 0.5-1.5mm, and is preferably in the range from 0.75-1.5 mm.

Since the hydraulic diameter cannot be known until the lumen is formed,a membrane with a lumen having an average diameter must be made whichwill match the hydraulic diameter sought. The average diameter of thelumen is defined as: [(the outside diameter of the membrane, “o.d”)minus {twice (the thickness of membrane from surface to crest of lobe,“hc”) plus (the thickness of membrane from surface to nadir of a valleybetween successive convex arcs, “hv”)}] divided by 2, which may bewritten as: [o.d.−2(hv+hc)]/2 (see FIG. 7)

The outside diameter of the cable is preferably in the range from 0.75mm-2.5 mm; the average diameter of the lumen chosen to match a desiredHD will depend upon the diameter of filaments used, the type of polymerused, the desired wall thickness of the membrane and other factors. Theweave of the braid is such that liquid membrane polymer embeds more than99% of the filaments including those in contact with the surface of thecable.

In practice, after coagulation of the dope, the cross-section of themembrane's lumen is slightly, from 1-10%, greater than the cross-sectionof the cable on which it was formed. Further, the monofilaments of theopen weave occupy less than 50% of the cylindrical surface of the cable,preferably less than 25%, depending upon the diameter of the filamentsand the spiral angle of the weave. A typical tubular braid, outerdiameter 2.0 mm, which is circumferentially continuous, collapses into ashapeless braid if the cable upon which the braid is woven, is removedbefore the membrane is formed. Such a braid has no shape, and has nomeaningful cylindricity.

Typically, the braid when woven and supported on the cable, has acylindricity of>0.8. The braid is woven using from 6-24 filaments of thesame diameter, each typically having a density in the range from 0.9-1.5g/ml and a denier in the range from 25-250 denier, approximatelyequivalent to a diameter of from 50-160 μm. If there is an overridingreason to do so, a mixture of coarse “reinforcing” and relatively fine“mesh” filaments may be used.

The braid, as a monolayer of monofilaments snugly overlying the cable,in turn, provides support for a tubular film of polymer film formedafter the braid is infiltrated with dope and coagulated. The membrane soformed has unexpectedly greater peel strength, durability and a higherbubble point than a multifilament braid of comparable nominal diameter.The term “monolayer” refers to a single layer of filament in which layerthe only filaments overlying one another are those at locations wheretwo filaments are interlaced over one and another. Interlaced filamentsare movable relative to one and another until embedded in coagulatedpolymer. Except in some instances in which the open weave is relativelyclosely woven and the thickness of the membrane is only slightlygreater, that is, <5% greater than twice the diameter of the reinforcingmonofilament used, the volume of the monolayer of filament occupies aminor portion (that is<50%) of the annular volume of the membrane.

A typical monofilament-reinforced membrane has a wall thickness in therange from about 0.2-0.6 mm generated by maintaining a ratio of hv/hc>1,preferably in the range from 1.2-5.

There is no acknowledgment in any reference that producing an open weavebraid of the claimed diameter, with monofilament having the aforestateddiameter, while the braid is supported on a scalloped cable chosen tomatch the desired diameter of the lumen of the membrane to be formed,requires modified spinning machinery not known by the inventors to beavailable in the prior art, anywhere.

The hollow fiber asymmetric microporous membrane (“membrane”) producedas described herein has a relatively thick wall of polymer film,preferably>0.2 mm but<0.5 mm thick, within which the monolayer isembedded near the lumen. A membrane so produced avoids problemsassociated with braids woven with any multifilament yarn irrespective ofthe polymer used for the membrane.

A tubular braid having a unique, open, weave (referred to as “open-weavebraid”) is woven with multiple monofilaments each in an axiallylongitudinal helical configuration, in a monolayer with arhomboid-shaped or diamond pattern (as in a playing card), as shown inFIG. 1. Preferably from 12 filaments are used, depending upon thethickness (denier) of the filament and the diameter of the tubularmembrane to be woven. Preferably the braid for the novel membrane iswoven from filaments of the same diameter. A preferred braid is wovenwith filaments of a single denier in the range from 80-120 denier andthe membranes formed have a nominal outer diameter in the range from0.85-2.5 mm using a cable having a diameter in the range from 0.5-2.0mm, the wall thickness of polymer being in the range from 0.2-0.6 mm,referred to as a thin polymer film.

The monolayer can only be embedded so as to define, in combination withcoagulated polymer surrounding the monolayer, the lumen of the membrane.The position of the embedded monolayer relative to the polymer above it,can only be manipulated by changing the nominal outer diameter of themembrane.

More specifically, the monolayer of monofilament in the membraneprovides rhomboid-shaped zones corresponding to the open-weave braid.The width of each zone is defined by the arcuate (because it iscircumferential) rhomboid area within the recurring area confined byintersecting filaments. Since the filaments typically occupy less than25% of the cylindrical area of the cable, the resulting relatively largefilament-free radial zones of polymer formed, provide better (greater)filtration into the lumen than that obtained with a braidedmultifilament yarn in a comparable prior art membrane. The membrane hasan adhesive strength>10 Kgf/cm², a bubble point>2 bar, and a percentrejection of 0.03 μm polystyrene bead>90%, and more preferably themembrane has an adhesive strength>15 Kgf/cm², a bubble point>4 bar, anda percent rejection of 0.03 μm polystyrene bead>95%.

Further, high permeate rates are maintained for longer than in acomparable multifilament membrane.

A process is described for embedding a monofilament, tubular open-weavebraid of monofilaments comprising, weaving an open tubular braid havinga recurring rhomboid pattern of synthetic resinous monofilaments in thedenier range from 25-250, directly over the surface of, and snuglycontacting a core cable (“cable” for brevity) of poly vinyl alcoholplasticized with from 5-20 weight percent of plasticizer, the cablehaving a scalloped profile with an average diameter chosen to provide alumen of desired average diameter in the range from 0.5-2.0 mm; coatingthe tubular braid with a membrane-forming dope in a coating nozzle untilthe dope infiltrates into an area below the surface of filamentsoverlying the cable to form the lumen; pulling the cable and braidtogether through the coating nozzle; coagulating the dope to form asemipermeable thin polymer film in a membrane embedding the braid as amonolayer which, together with polymer surrounding the braid, definesthe lumen of the membrane; washing in hot water until at least 99% ofthe plasticized PVA is removed, further washing with an oxidizing agentchosen from sodium hypochlorite (NaOCl), hydrogen peroxide and potassiumhypochlorite (KOCl) to make a membrane which has a total organic carbon(“TOC”) of<0.5 ppm without damaging the lumen of the membrane formed.

From the foregoing it will be evident that having first made thediscovery that “whiskering” and “fuzz” (associated with multifilaments)were the root cause of membrane leakage, what still remained to bediscovered was (i) how to make the membrane having the diameter stated,(ii) how to position a monolayer of monofilament so as to define thescalloped lumen of a thin membrane, (iii) how to control the thicknessof membrane polymer both above and beneath the monolayer of braid, (iv)how to melt-extrude a PVA cable having the stated diameter with physicalproperties suitable for the task at hand, and (v) how to rid themembrane of essentially all, that is>90% of trapped air and plasticizedPVA and contaminants so that the TOC of the membrane is preferably below3 ppm.

The complex nature of the solution to the problem cannot reasonably bedisregarded in any attempt to use the teachings of the foregoingreferences as an enabling disclosure.

BRIEF DESCRIPTION OF THE DRAWING

The foregoing and additional objects and advantages of the inventionwill best be understood by reference to the following detaileddescription, accompanied with schematic illustrations of preferredembodiments of the invention, in which illustrations like referencenumerals refer to like elements, and in which: FIG. 1 is a frontvertical perspective view, of a braid supported on a tri-lobe PVAsupport, referred to as a cable, prior to being infiltrated with a dope.

FIG. 2 is a photomicrograph of a sheathed cable at a 50× magnificationshowing the cable 30 over which twelve monofilaments 12, each 100denier, are spirally woven at an angle of about 35° to the longitudinalz-axis of the cable.

FIG. 3 is a schematic cross-sectional view of the membrane in the x-yplane at right angle to the axial z-axis showing filaments of amonolayer of braid woven with 12 filaments some of which contact thelobes of the tri-lobe cable.

FIG. 4 is a perspective isometric staggered cross-sectional viewschematically illustrating the filaments covering the tri-lobe cablewhich is to be dissolved, leaving some small clearances between thecable and filaments filled with coagulated polymer, so as to form thelumen of the membrane after coagulation.

FIG. 5 is a schematic cross-sectional view of the membrane in the x-yplane at right angle to the axial z-axis showing filaments of amonolayer of braid woven with 12 filaments some of which form a boundaryof a double-barreled lumen formed by the opposed lobes of a twin-lobedcable.

FIG. 6 is a schematic cross-sectional view of the membrane in the x-yplane at right angle to the axial z-axis showing filaments of amonolayer of braid woven with 12 filaments which together form aboundary of a four-lobed, or quadric-lobed, lumen formed by afour-lobed, or quadric-lobed, cable.

FIG. 7 illustratively duplicates a photomicrograph showing macrovoidsaround the periphery of the lumen where trapped air was released, andcross-sections of the filaments which were displaced within the polymerfilm when the membrane was forcibly sectioned.

FIG. 8 illustrates the process steps for forming the membrane startingwith a flexible, plasticized scalloped PVA cable sheathed in an openweave braid, to produce a membrane which is wound on a spool in a winderbath.

FIG. 9 illustrates the transfer of a bundle of fibers cut from the spooltaken from the winder bath, which bundle is given a finishing treatmentwith dilute aqueous sodium hypochlorite (NaOCl).

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Supporting the braid as it is woven on the cable:

Referring to FIG. 1, there is shown a sheathed cable “SC” comprising ascalloped cable 30 over which is spirally woven a braid 10 comprisingmonofilaments (or “filaments”) 12. The filaments 12 are made from asynthetic resinous material (“filament polymer”) which is insoluble inpermeate to be filtered through the membrane to be formed. The filamentpolymer is preferably selected from the group consisting ofpoly(vinylidene difluoride) (“PVDF”), polycarbonate, polystyrene,polyester, polyolefin, polyamide, poly(methyl methacrylate), poly(vinylchloride) and glass fiber. Filaments 12, typically all of the samedenier, are wound at the same spiral winding angle greater than 20°,preferably in the range from 20°-60° relative to the longitudinal axisof the mandrel by a custom-built braiding machine using twelve (12)cones modified to hold and discharge a filament less than 250 μm indiameter; some filaments, typically alternate filaments, are wound at anaxially, oppositely directed angle from each other to provide interlacedfilaments in what is commonly referred to as a diamond weave. A largewinding angle indicates the filament is wound closer to the x-y plane (atransverse orientation); a small winding angle indicates the filament ismore axially aligned as it is wound. The tubular shape of the braid isprovided by contact of filaments 12 at points which may create smallclearances or spaces 14 on the surfaces of the lobes after the braid iswoven.

FIG. 2 is a photomicrograph of a portion of the sheathed cable showinghow twelve (12) filaments 12 snugly embrace the cable 30, often with theclearances 14 less than 50 μm, between the underside of the filamentsand curved surfaces near the contact points on the cable; and showingmuch larger spaces 14′ between the underside of the filaments and othercurved surfaces away from and on either side of the contact points. Thesmall clearances, if any, result from relaxation of the woven braidafter it is removed, and prior to coating it with polymer 11. Thepolymer 11 infiltrates all such spaces 14 and 14′.

The location of the braid relative to the cable, in the polymer definingthe lumen formed when the cable is dissolved, is thus fixed. Itslocation relative to the wall thickness of the membrane, for a stateddiameter of the lumen, can only be manipulated by increasing ordecreasing the nominal outer diameter of the membrane.

FIG. 3 illustrates the positioning of the filaments in the membranecovering the cable viewed in the vertical x-y plane. Some of the twelvefilaments contact, or nearly contact the crests of the lobes 21 of thelumen 16; other filaments are in contact with each other at points 15where they overlie one and another at an intersection; still otherfilaments are close to their intersection points above the valleys 19between contiguous lobes. The coagulated polymer film 11 provides arelatively deep tubular layer essentially free of macrovoids. Aphotomicrograph of a sliced cross-section of the tri-lobe (orthree-lobed) membrane showing details of its physical structure isprovided in FIG. 7. The integrity of the tubular layer overlying andembedding the filaments, is the key to the excellent performancecharacteristics of the tri-lobe membrane, as evidenced by the datapresented in Tables 1 & 2 below.

FIG. 4 illustrates the membrane 20 as it was formed on the tri-lobecable 30 which was subsequently dissolved. Some of the contact points ofthe filaments and the upper surfaces of the lobes of the cable, showthat some filaments get slightly pulled away from the surfaces, leavingthe small clearances 14 due to relaxation of the filaments after thebraid is woven, and due to forces encountered in the environment of thecoating nozzle. Other filaments leave spaces 14′ because the filamentsare above the valleys 19. All these spaces get filled with polymer.

FIG. 5 illustrates a double-barreled membrane 42, in which the polymeris coagulated to form the tubular film 11 with a lumen 43 having twindiametrically opposed lobes 44 in the x-y plane, on a double-barreledcable which has been dissolved, in a manner analogous to the membraneformed as described for FIG. 3. When such a lumen of maximumcross-sectional area is desired, irrespective of the number of lobes,but particularly with a maximized twin-lobe lumen, with lobes positionedside-by-side and diametrically opposed, the integrally extrudeddouble-barreled cable used to produce the lumen, is necessarilyconnected by a small thickness of PVA to maintain the integrity andstrength of the cable when it is extruded; and, the lobes are not seento be tangential relative to each other. The lobes are spaced apart bythe thickness “s”, which results in the terminal ends of the adjacentarcs also being similarly spaced apart. The distance “s” is preferablyless than 25% of the diameter of the circles (and arcs) defining thelobes.

FIG. 6 illustrates the membrane 45, in which the polymer is coagulatedto form the tubular film 11 with a lumen 46, having four radiallyequispaced lobes 47 in the x-y plane, on a four-barreled cable which hasbeen dissolved, in a manner analogous to the membrane formed asdescribed for FIG. 3.

FIG. 7 is a photomicrograph of a sliced cross-section of a membranehaving a tri-lobe lobe lumen formed by coagulated film 11, showingmacrovoids or pits 17 where small air bubbles, in the range from 1-100μm, more typically from 5-50 μm, were trapped near the boundary of thelumen, mainly in the valleys between the lumen's interconnected lobes,when the wall of the membrane was formed. The trapped air was thenreleased, when the cable supporting the reinforcing braid was dissolved,leaving the pits as the air bubbles' fingerprints. The generally smallermicrovoids 18, typically in the range from 5-20 μm, around thefilaments, resulted when the filaments were moved in the matrix polymerby impact of the sectioning blade. A sub-circumferential layer ofperipheral microvoids 22, typical for a membrane formed with PVDF/NMP,is formed when the liquid polymer coagulates. This peripheral layer isshown as dots, directly below the circumference, in FIG. 7.

It is seen that the filaments 12 overlap at intersections 15 of theembedded filaments 12 of the braid over which the membrane 20 is formedon the cable 30. When the braid of filaments 12 is woven over the cable30, there are clearances 14 and 14′, which range in diameter from 5μm-0.1 mm or more, depending upon how closely the surfaces of thefilaments, or whether the filaments overlie a valley 19 betweencontiguous lobes. The amount of such clearance depends upon the numberof lobes of the cable, its diameter and other factors. Polymerinfiltrates the clearances and when coagulated forms the film 11.

Though an open-weave filament is expected to be weaker than a prior artweave of multifilament, the resulting open-weave membrane retains itstubular configuration without collapsing under suction pressure exertedduring filtration, and has excellent peel strength of at least 15kgf/cm². The braid, with the cable removed, has an airpermeability>(greater than) 100 cc/sec/cm² at a ΔP of 1.378 kPa. Themonofilament construction of the braid ensures stability and a minimalmoisture regain, much less than that of a comparable multifilamentbraid; and the unique open-weave of the braid 10 makes it unnecessary topreshrink it to ensure its stability.

Hollow Fiber Membrane and Process for Making it:

Referring to FIG. 8, there is schematically shown a flowsheet for theprocess of making a braided PVDF membrane wherein a sheathed cable SC(tubular open-weave braid 10 of twelve filaments 12, each 100 denier,covering cable 30) is fed from braid un-winder 23 over guide rolls 24and 25 to a coating nozzle 50.

The dope is prepared by mixing from 10˜30% by weight of the PVDF withfrom 70˜90% by weight of N-methylpyrrolidone (NMP) in a dope tank 40 inwhich the dope is blanketed with an inert gas, preferably nitrogen, fromcylinder 41. The dope may be prepared by any conventional method and mayinclude suitable additives, if needed. The dope is prepared by mixing 20wt % of PVDF (product name: Solef 1015) and 80% of N-methylpyrrolidone(NMP) at 60° C. at a temperature in the range from 30° C.˜100° C.,preferably 40° C.˜70° C. so that the viscosity of the dope is in therange from 30,000 cps˜60,000 cps at 30° C.

The covered cable SC is fed from unwinder 23 and over guide rolls 24 and25 to a coating nozzle 50. Cable 30 is an extrudate of PVA plasticizedwith 10% glycerine. The cable has a diameter of 0.75 mm; the filamentsare woven at a weave angle of 35° to sheathe the cable 30 with theopen-weave tubular braid.

The dope is metered through coating nozzle 50 to produce a 400 μm thickfilm with the braid embedded in the bottom of the film. The dope is thencoagulated at a temperature of 30˜50° C. in an aqueous coagulation bath60 and fed over guide rolls 61 and 62 to a cleaning bath 70. The washwater in a first cleaning bath 70 is maintained at a temperature of40˜80° C. for from 0.5˜1.5 min to dissolve and wash out the residual NMPfrom the membrane.

The membrane 20 on cable 30 leaves over guide roll 73 and is cleaned ina second cleaning bath 76 maintained at a temperature of 60˜80° C. afterwhich the cleaned membrane leaves under guide roll 74, and is capturedon a winder 80 in a winder bath 81 of diluted 50% aqueous glycerine. Thediluted glycerine prevents an upper layer of wrapped membrane fromsticking to a contiguous lower layer. The purpose of the winder is tostore the washed membrane and the cable still supporting it, until themembrane can be cut into short sections, approximately the lengthdesired for building a desired module, and freed from the plasticizedPVA cable.

Accordingly, as illustrated in FIG. 9, a bundle 26 of about 2500 cutsections of SC each about 2.5 m long, is hung vertically in acable-dissolving tank 27 into the top of which is introduced 60˜80° C.hot water until the bundle is submerged. As the plasticized PVAdissolves, it flows downwards through the lumens of the membranesbecause the density of a saturated solution of PVA is about 1.33. Thewater contaminated with PVA collects in the bottom of the tank asdissolved PVA and is removed.

When the concentration of PVA in the wash water leaving tank 27 is<0.5%the bundle 26 is removed from the tank 27. Because too many pores of themembranes are still clogged, the bundle 26 is mounted horizontally in asecond pore-cleansing tank 28 into which an aqueous solution of from0.1-0.5% NaOCl solution at from 20° C.-80° C., preferably from 40°C.-60° C., is introduced to remove the remaining PVA and othercontaminants which would restrict flow of permeate into the lumen. Thesolution is continuously recycled by pump 29 through piping 31overnight, then drained through drain piping 33. The bundle ofmembranes, each with a lumen having a diameter of 0.8 mm, now free ofPVA and other contaminants which clogged the pores of the membrane, istransferred to a module-building facility.

The monolayer of braid 10 is thus embedded in coagulated PVDF film 11which has excellent permeability and is essentially insoluble in water.The polymer acquires a pattern of rhomboid areas 13 (see FIG. 1)generated by the embedded braid, each area bounded by monofilaments 12.These areas 13 provide direct unobstructed radial passage of permeateinto the lumen 16. “Radial passage” refers to the path of permeationfrom the surface of the membrane 20 to the lumen 16. To control thepattern, and also to provide greater strength, the filaments 12 may besonically or thermally welded at intersections 15. The intersections 15are the only locations where the filaments overlap and contact eachother. The area of each of the zones depends upon the openness of theweave, the diameter of the filament used and the spiral angle of theweave.

The diameter of plasticized PVA cable with a scalloped periphery ischosen according to the desired diameter of the lumen (inner diameter ofthe membrane). Typically, the cable, whether one or more, has an averagediameter in the range from 0.1˜1.8 mm, preferably 0.5˜1.5 mm, to providea braid reinforced membrane having an average wall thickness in therange from 0.2-0.5 mm. The resulting lumen is non-circular and the wallthickness of the membrane formed is non-uniform.

Preferred braided membranes have a bubble point of at least 2 bar. Thenovel membrane has an adhesion strength of more than 15 kgf/cm²typically from 12 to 20 kgf/cm².

The open-weave monolayer braid 10, embedded in the polymer film 20,unexpectedly provides excellent permeability and resistance to damage.The embedded monolayer eliminates a “whiskering” problem common tobraids woven with one or more multifilament yarns.

The recurring open areas in the open-weave braid providecircumferential, interconnected rhomboid or diamond-shaped loops 13lying in the vertical (as shown in FIG. 1) axial (z-axis) direction,and, no filament is deployed circumferentially in a generally planarcircle in the x-y plane. Since there is no constriction in the radialdirection, not only the braid but also the membrane may be biaxiallydistended under sufficient internal fluid pressure prior to beingdamaged. By “biaxially distended” is meant that the open-weave braidallows not only substantial longitudinal extension of the membrane, suchas might occur during backwashing, but also allows substantial radialdistension of the membrane which typically does occur duringbackwashing. As would be expected, the longitudinal expansion of theembedded braid is much less than that of the braid itself, but muchgreater than that of a prior art multifilament braid coated with thesame polymer.

The rhomboid pattern 13 is retained when the intersections of filaments12 are welded. The pattern may be more close-knit (not shown) so that itprovides a membrane with smaller radially open passages to the lumen,each of smaller area relative to an area defined by the relativelyloosely knit braid (shown).

Prior art braids, woven with multifilament yarn, have at least some ofthe yarn forming an essentially continuous circle in the x-y plane, thusresulting in constricting any radial distension of the braid. Prior arttubular multifilament braids are therefore inelastic in the radialdirection. This constricting property is retained even when the braid isembedded in an elastic polymer film. Pressure exerted from within aprior art multifilament tubular braided membrane, cannot and does notincrease the diameter of the braid, thus making it susceptible todamage. In contrast, in addition to the longitudinal extension one wouldexpect of the elastic properties of a polymer-embedded, open-weavemonofilament reinforced tubular membrane, such properties allow themembrane to expand or distend radially, when sufficient pressureprovides a radial distending force. Consequently, a relatively higherpressure than normally used, sufficient to distend the membrane butinsufficient to damage it, may be used to backwash the membrane.

Example 1

Duplication of feeding a monofilament braid as described in the Yoon etal publication No. US 2009/0206026:

A monofilament braid of 100 denier (0.1 mm) nylon monofilament wasspirally woven on a 1.0 mm cable of metal wire, alternate filamentsbeing woven at opposed spiral angles of 30°. The sheathed cable wasplaced on a 2.54 cm diameter rubber roller rotating at 30 rpm. The braidwas crumpled on the cable and could not be advanced. The speed of theroller was reduced to 15 and then 5 rpm. In no case was the braidadvanced without damage. The roller speed was then increased to 40 rpm.The braid was crumpled.

Example 2

The following three grades of PVA available from Kuraray were each meltextruded in a single screw Hankook Model M-65 extruder fitted with a 65mm diameter screw having a length/diameter ratio of 22. The barreltemperature is 195° C. and the die temperature 160° C. The die isprovided with 18 scalloped through-apertures (holes) and the averagediameter of each aperture is 1.6 mm. The air quenching length for thePVA cable is 2 m in 25° C. air for 2 seconds. The drawing ratio is1.5:1.

Extrusion Temp 195° C. Fully hydrolyzed (F-05 and F-17) degradesIntermediate hydrolyzed (M-17) degrades Partially hydrolyzed (P-24,P-20, P-17 and P-05) degrades

Example 3

A braid is formed by weaving 12 filaments, each of 100 denier nylon, ata spiral angle of 35° over a plasticized PVA cable having a diameter of0.75 mm using a custom-built braiding machine. The sheathed cable ispulled through a coating nozzle into which a dope, prepared as describedabove to have a viscosity of 43,000 cps at 30° C., is flowed at anoutput rate of 11 g/min. The dope infiltrates the braid, coats the cableand embeds the braid. The membrane is coagulated in a water bath at 45°C. and washed as shown in FIG. 5. The wall thickness of the membrane is400 μm the braided monofilaments forming a monolayer around the lumenwhich has essentially a little larger diameter than that of thedissolved cable, namely 0.8 mm, because PVA cable is swollen in thecoagulation bath and cleaning bath before the membrane finishes itscoagulation. The cross-section of the braid is schematically illustratedin FIG. 3.

The physical properties of the membrane made in Example 3 above, aregiven in Table 1 below.

Example 4

In a manner analogous to that described in Example 3 above, a dope,prepared as described above to have a viscosity of 43,000 cps at 30° C.,is flowed at an output rate of 16 g/min a braid is woven at the samespiral angle, over a cable having a diameter of 1.1 mm using 12 nylonmonofilaments, each of 100 denier (0.1 mm) to yield a membrane with a1.25 mm lumen, and a nominal outer diameter of 2.05 mm.

Example 5

In a manner analogous to that described in Example 3 above, a dope,prepared as described above to have a viscosity of 43,000 cps at 30° C.,is flowed at an output rate of 19 g/min a braid is woven at the samespiral angle, over a tri-lobe cable having a average diameter of 0.85 mmusing 12 nylon monofilaments, each of 100 denier (0.1 mm) to yield amembrane with the average diameter of the lumen 0.93 mm, and a nominalouter diameter of 1.85 mm.

Comparative Example

In a manner analogous to that described in Example 3 above, a braid iswoven at the same spiral angle, without using a cable, with twenty fourPET multifilament yarns each 300 denier/96 filament (a single filamentis superfine, about 3 denier) and having an inner diameter of 0.85 mm;the braid was embedded in the same polymer solution to provide a wallthickness of 650 μm (0.65 mm, but the membrane film thickness is about100 μm).

Evaluation of Physical Properties:

1. Water Permeability

(i) A membrane having a length of 200 mm is folded in half and insertedin an acrylic tube having a diameter of 10 mm and a length of 100 mm. Atone end of the tube, the membrane, near both of its ends is sealed withepoxy (or urethane) leaving the lumen in each end open. The other end ofthe tube is left open and mounted in a water permeability testingapparatus.

(ii) Distilled water held under pressure, is discharged from apressurized vessel into the tube at a pressure of 1 bar (14.7 psig or100 kPa) so as to force water through the wall of the membrane anddischarge the permeate into a collection beaker. The water permeabilityis obtained by weighing of the permeate collected over a specifiedperiod of time.

2. Adhesion Strength:

(i) a fiber 50 mm long is inserted for 10 mm of its length near one end,into a 10 mm inside diameter polyurethane tube 50 mm long.

(ii) the 10 mm of fiber in the polyurethane tube is potted using epoxy(or urethane)

(iii) 10 mm of the other end of the fiber is wrapped with paper so asnot to damage it and the wrapped end is inserted into one of the jaws ofan Instron (UTM) tensilometer, the gage length of which was set at 70mm. Any material providing suitable gripping without damaging themembrane may be substituted for the paper. When the tube is secured inthe other jaw of the machine, the fiber is to be taut so as not to besuddenly elongated when the Instron is in operation.(iv) The crosshead speed was 50 mm/min, The maximum tensile stress isdivided by its unit area, so the maximum tensile stress is registered asthe adhesion strength.

The average elongation at break is registered as the elongation.

3. Bubble Point

(i) Use the same membrane sample prepared for the water permeabilitytest, mount the tube in the water permeability testing apparatus, thenimmerse the tube including the membrane in a water bath.

(ii) The testing apparatus is connected to a source of nitrogen underpressure and, with the membrane immersed, the tube is pressurized withnitrogen.

(iii) Adjust the pressure to the tube in stages with a pressureregulator, through a range from 0 bar (atmospheric) to 8 bar, holdingthe pressure in increments of 0.5 bar for 60 seconds.

(iv) Record the pressure when a nitrogen bubble forms on the surface ofthe membrane, or alternatively, the pressure at which the membraneruptures.

(v) The recorded pressure is the bubble point.

4. Percent (%) Rejection of Particles

UV [using a Perkin Elmer Lambda 25 UV/vis spectrometer]

(i) preparing two strands of the hollow fiber membrane having a lengthof 100 mm

(ii) inserting the membrane in an acrylic tube having a internaldiameter 10 mm and a length of 100 mm; sealing one end of the membranewith paraffin (or urethane). The other end of the membrane is potted inthe acrylic tube to prepare a sample.

(iii) mounting the sample in a water permeability testing apparatus

(iv) preparing a feed solution for measuring rejection ratio, asfollows:

A styrene bead dispersion was prepared by mixing polystyrene beadshaving a size of 0.03 μm in thrice distilled (or reverse osmosis) waterto provide a dispersion having a concentration of 0.05% polystyrenebeads, with enough surfactant to prevent styrene beads from stickingtogether, and agitating the mixture for 1 hr.

(v) pouring the dispersion into a pressure vessel and under a pressureof 0.5 bar, forcing the polystyrene beads through the membrane and

(vi) collecting the permeate over a period of 1 minute.

(vii) setting a reference line for the thrice distilled water using aUV-Visible spectrometer and measuring the absorbance of the feedsolution, then measuring the absorbance of the permeate collected.

(viii) The % rejection can be obtained by using a UV-Visiblespectrometer and be calculated by the following formula:Rejection (%)=(1−C _(permeate) /C _(feed))*100

C_(feed): absorbance of the feed solution:

C_(permeate): absorbance of a sample passed through the membrane

A rejection ratio 90% or more is deemed useful and pore size of themembrane can be estimated indirectly by using dispersions of beadshaving various diameters ranging from 20 nm-100 nm.

TABLE 1 Water Adhesion Elongation Bubble Outer Diam. Inner Diam.Permeability Strength at break Point Pore Size Rejection (mm) (mm)(LMH/Bar) (Kgf/cm²) (%) (bar) (μm, SEM) Ratio (%) Example 3 1.6 0.8 80017 51 6 0.03 98 Example 4 2.05 1.25 800 18 52 5.5 0.03 97 Example 5 1.80.93 750 15 50 7 0.03 97 Comparative 2.1 0.85 600 12 31 1.5 0.04 95Example

It is evident from the Table 1 above, that the pore sizes for each ofthe membranes are essentially the same, as one would expect. However,the water permeability of the membrane with multifilament yarn in thebraid, is only 75% of the membrane with the monofilament braid, itsbubble point is lower than 33%, and its elongation at break is 66% lowerthan that of the membrane with the monofilament monolayer braid.

Further, the data for the membrane with the tri-lobe lumen, Example 5,show a bubble point about 20% higher than those obtained with membraneshaving circular lumens, Examples 3 and 4.

Weight Advantage of Monofilament Braid Membrane

Equal lengths (1 m) of a membrane made as described in Example 3, 4 and5, and a membrane made with a multifilament braid described in theComparative Example above are dried so as to contain less than 1% byweight of water. Each membrane was then soaked in 30% glycerine solutionfor 24 hours and dried at 30° C. convection oven for 4 hours andweighed. The membranes were thereafter soaked in water for 24 hours,then weighed. The results are given in Table 2 below.

TABLE 2 Membrane weight Membrane weight after 30% Membrane weight afterdrying Glycerine treatment after water intake (g/m²) (g/m²) (g/m²)Example 3 108 181 360 Example 4 115 186 356 Example 5 119 193 371Comparative 255 385 516 Example

It is evident from the results above, that the multifilament braidretains more than double the amount of glycerine, and about 43% morewater than the monofilament membrane. Such increased weight is magnifiedwhen several thousand membranes are assembled in a module, making itmore difficult to insert and remove modules in a purification system.

Particularly with respect to the efficacy of removal of the plasticizedPVA in the membranes before they are assembled into modules and placedin service, all three membranes test routinely for<0.5 ppm TOC beingtypically<0.3 ppm TOC using the prescribed KWWA (Korea Water andWastewater Works Association) F 106 test. This confirms that essentiallyall the plasticized PVA has been removed.

Having thus described the monofilament membrane having a scallopedlumen, and the process for making the membrane, in detail, andillustrated both with specific examples of the best mode of each, itwill be evident that we have provided an effective solution to anunrecognized problem. It is therefore to be understood that no unduerestrictions are to be imposed, and our invention not restricted to aslavish adherence to the details set forth herein.

REFERENCE NUMERALS SC sheathed cable 33 drain piping 10 braid 11coagulated film 12 filaments 36 double-barreled lumen 13 rhomboid spaces14 & 14′ filled spaces at/near contact points 15 intersections ofoverlapping filaments 16 lumen 40 dope tank 46 17 macrovoids 41 nitrogencylinder 18 smaller macrovoids 42 double-barreled membrane 19 valleysbetween tri-lobes 43 twin-lobed lumen 20 tri-lobed membrane 44twin-lobes/ twin-lobed lumen 21 lobes of tri-lobe lumen 45 quadri-lobedmembrane 46 quadri-lobed lumen 23 unwinder 47 quadri-lobes/quadri-lobedmembrane 24 guide roll 25 guide roll 26 bundle of SC 50 coating nozzle27 PVA dissolving tank 60 coagulation bath 28 pore-cleansing tank 61guide roll 62 guide roll 30 tri-lobed cable 70 first cleaning bath 31piping 73 guide roll 80 winder 74 guide roll 81 winder bath 76 secondcleaning bath

We claim:
 1. A continuous braid support of arbitrary length for a braidfor a semipermeable polymer film, the support comprising, anaqueous-dissolvable cable of poly(vinyl alcohol) (“PVA”) plasticizedwith a plasticizer in an amount sufficient to yield a homogeneous,solid, flexible extrudate having a cross-section with a scallopedcircumference comprising plural interconnected circumferential,outwardly convex segments or lobes, the extrudate having an averagediameter in the range from 0.75 mm-2.5 mm, a density±10% of the densityof PVA, and sufficient strength to provide an elongated, continuous,solid, unitary support for a braid of monofilaments only, woven on thesurface of the extrudate; braid a surrounding the support comprising,from 6 to 24 separate monofilaments each in the range from 25-250 denier(gm/9000 meters), woven in a spiral open weave to provide contiguousrhomboid areas bounded by monofilaments , the spiral weave being wovenat an axially oppositely directed spiral angle in the range from 20°-60°from the longitudinal axis so as to be free of a restrictingcircumferential filament.
 2. The support of claim 1 wherein theplasticizer is present in an amount in the range from 5- 20% by weightand is selected from the group consisting of poly(ethylene oxide) (PEOor PEG), poly(propylene oxide) (PPO or PPG), ethylene oxide capped PPO,sorbitol, glycerol, ethylene glycol, poly(vinyl pyrrolidone),pentaerythritol, 1,4-monoanhydrohexitol, 1 ,4-3,6-dianhydrohexitol andcopolymers of poly(vinyl acetate).
 3. A process for embedding amonofilament, tubular open-weave braid of monofilaments in a membranepolymer, comprising, weaving an open tubular braid having a recurringrhomboid pattern of synthetic resinous monofilaments in the denier rangefrom 25-250, directly over the surface of and snugly contacting a corecable of poly(vinyl alcohol) (PVA) plasticized with from 5-20 weightpercent of plasticizer, the cable having an average diameter in therange from 0.75-2.5 mm, and chosen to provide a lumen having a scallopedcross-section; coating the tubular braid with a membrane-forming dope ina coating nozzle until the dope infiltrates into an area below thesurface of filaments overlying the cable to form the lumen; pulling thecable and braid together through the coating nozzle; coagulating thedope to form a semipermeable membrane embedding the braid as a monolayerwhich, together with polymer surrounding it, defines the lumen of themembrane; washing in hot water until at least 99% of the plasticized PVAis removed, and, further washing with an aqueous oxidizing agent chosenfrom sodium hypochlorite (NaOCl), hydrogen peroxide and potassiumhypochlorite (KOCl) to make an asymmetric membrane which tests foreluted water having a total organic carbon (“TOC”) of<0.5 ppm withoutdamaging the lumen of the membrane formed.
 4. The process of claim 3wherein the semipermeable membrane has a wall thickness in the rangefrom 0.2-0.6 mm thick.
 5. The process of claim 3 wherein theconcentration of NaOCl in the aqueous oxidizing agent is in the rangefrom 0.1-0.5% NaOCl and its temperature is in the range from 20° C.-80°C.